Audio amplifier circuits have a somewhat unique problem in that they are required to translate electrical signals in the electronics world into a moving pressure wave in the physical world. A speaker is used to translate currents from the audio amplifier circuits into a moving pressure wave. The pressure wave is created by using the speaker to force a mass of air to move along an axis of movement towards the listener. The particles in the forced mass of air collide with the air particles that are in front of the direction of movement causing compression of the air, or increased pressure. Compressed air has a higher number of molecules than uncompressed air such that there is an increase in the number of collisions with adjacent air particles. The compressed air particles then collide with the particles in front of them, which collide with the particles in front of them, and so on, creating a moving pressure wave. The air eventually is received into the inner ear, which senses the fluctuations in the air pressure, and translates the fluctuations into electrical signals that the human brain can understand. Simply, the rate of vibration of the air is translated by the brain into pitch (or frequencies), while the amplitude of the fluctuations (or sound pressure level) is translated by the brain into volume.
Speakers have a rated efficiency that simply maps the speaker's ability to translate power into sound pressure levels. The power levels that are necessary for an audio amplifier to effectively drive a speaker sufficient for a typical listener is related to a number of factors including the size of the speaker, the efficiency rating of the speaker, and the size of the listening area. For example, a pair of headphones operates like small speakers that are located in close proximity to the ear. Since the distance between these small speakers and the inner ear is very close, the efficiency of delivery of sound from these small speakers is very high. In contrast, a speaker that is located in a room several feet from the listener's ear does not deliver sound as efficiently since a greater mass of air must be moved to reach the outer ear, and then only a portion of that mass is received in the inner ear. As a result of the physics of air movement and related issues, the total amount of power that is necessary to drive the speaker to acceptable volume levels in headphones is significantly smaller than similar perceived volume levels in speakers in a large room.
Typically, a transistor amplifier is used to drive each speaker. The available power that can be delivered to a speaker by a transistor amplifier is related to many factors including the class of operation. Although a variety of amplifier classes are described below, the most popular amplifier classes for audio amplifiers are: class A, class AB, and class D.
Class A amplifiers typically include transistors that are biased so that variations in input signal polarities occur within the limits of cutoff and saturation associated with the transistors. In a PNP transistor, for example, if the base becomes positive with respect to the emitter, holes will be repelled at the PN junction and no current can flow in the collector circuit. This condition is known as cutoff. Saturation occurs when the base becomes so negative with respect to the emitter that changes in the signal are not reflected in collector-current flow. By maintaining the transistors biased in this manner, with the DC operating point between cutoff and saturation, the current in the transistor flows during the complete cycle (360 degrees) of the input signal.
Class B amplifiers typically include transistors that are biased so that collector current is cutoff during one-half of the input signal. The DC operating point for this class of amplifier is set up so that base current (in a PNP or NPN style transistor) is zero with no input signal. When a signal is applied, one half cycle will forward bias the base-emitter junction and collector current will flow. The other half cycle will reverse bias the base-emitter junction and collector current will be cut off. Thus, for class B operation, collector current will flow for approximately 180 degrees (half) of the input signal. Class B amplifier have no wasted power when there is no input signal since the transistors are biased in cutoff. However, since the initial input signal must overcome the cutoff point of the transistors, class B amplifiers have a dead spot where the transistors are cut off that creates a distortion characteristic in the output signal known as crossover distortion. Crossover distortion results in a loss of fidelity, or faithful reproduction, relative to the input signal.
Class C amplifiers typically include transistors that are biased such that collector current flows for less than one half cycle of the input signal. By reverse biasing the emitter-base junction (in a PNP or NPN type transistor), the DC operating point of the transistor is below cutoff and allows only the portion of the input signal that overcomes the reverse bias to cause collector current flow. Although the efficiency rating of class C amplifiers is quite high, their fidelity is very poor.
Class AB amplifiers typically include transistors that are biased so that the collector current is zero (cutoff) for a portion of one cycle of the input signal. This can be accomplished by making the forward-bias voltage less than the peak value of the input signal. The base-emitter junction will then be reverse biased during one cycle until the input signal voltage exceeds the forward-bias voltages. The resulting collector current will flow for more than 180 degrees but less than 360 degrees of the input signal. As compared to the class A amplifier, the DC operating point for the class AB amplifier is closer to cutoff. Class AB operated amplifiers are commonly used as a push-pull amplifier (one circuit for the positive input cycle and one circuit for the negative input cycle) to overcome crossover distortion that commonly occurs with class B amplifiers.
Class D amplifiers typically include transistors that are operated as switches. When the switches are off, the current through the switch is zero. When the switch is on, the voltage across the switch is small and current is delivered through the switch. The audio input signal is translated into a series of pulses for which the width and time duration of each pulse is related to the instantaneous amplitude of the input signal. The current pulses are delivered to a low pass filter circuit that commonly consists of an inductor (L) and a capacitor (C). The low pass filter passes the average value of the pulses to the speaker. Class D amplifiers have a very high efficiency in delivering higher power to the speaker.